Когда я готовлю, я надеваю фартук, чтобы не испачкать рубашку.

It most naturally means when / whenever I cook in a general, habitual sense.

Because готовлю is present-tense imperfective, this clause describes something that happens regularly:

• Когда я готовлю, ... = When/Whenever I cook, ...

Depending on context, English might also say when I’m cooking, but Russian here sounds more like a repeated habit than one specific moment.

Russian uses the imperfective here because cooking is presented as an ongoing activity or a repeated situation, not as a completed result.

• готовить / готовлю = to cook, be cooking, cook regularly
• приготовить would focus more on finishing or completing something

So Когда я готовлю is about the process or habitual action of cooking, which is why imperfective is the natural choice.

Because Russian often repeats the subject in each clause, especially when the sentence has a clear two-part structure.

So:

• Когда я готовлю, я надеваю фартук...

sounds fully natural and clear.

Could you sometimes omit the second я? Yes, in some contexts:

• Когда я готовлю, надеваю фартук...

That is possible, but the version with both я is very normal and often better for learners to model.

This is a very common learner question.

• надевать / надеть = to put on an item of clothing
• одевать / одеть = to dress someone

So:

• Я надеваю фартук = I put on an apron
• Я одеваю ребёнка = I dress a child

A useful shortcut:

• you надеть something
• you одеть someone

That is why фартук goes with надеваю, not одеваю.

Because the sentence describes a habitual action: every time the speaker cooks, they put on an apron.

• надеваю = I put on / I am putting on, as a repeated or regular action
• надену = I will put on / I’ll put on, usually one specific future action

So here Russian uses imperfective to match the repeated meaning:

• Когда я готовлю, я надеваю фартук... = Whenever I cook, I put on an apron...

It actually is in the accusative case, but for many inanimate masculine singular nouns, the accusative looks exactly like the nominative.

So:

• nominative: фартук
• accusative: фартук

Because фартук is the direct object of надеваю, it must be accusative, even though the form does not visibly change.

Compare with a feminine noun, where you can see the change more clearly:

• рубашка → рубашку

Russian often leaves out possessive words like my, your, his, etc. when the meaning is already obvious from context.

Here, if the speaker is putting on an apron to avoid dirtying the shirt they are wearing, Russian does not need to say мою рубашку.

So:

• не испачкать рубашку very naturally means not to get my/the shirt dirty

If the speaker wanted to emphasize ownership, they could say:

• не испачкать мою рубашку

But in everyday Russian, that would often sound unnecessary.

Here чтобы introduces a purpose clause. It means something like:

• so that
• in order to
• so as to

So:

• я надеваю фартук, чтобы не испачкать рубашку means
• I put on an apron so that / in order not to get the shirt dirty

This is one of the most common uses of чтобы.

Because the subject is the same in both parts of the sentence.

The speaker:

• puts on the apron
• and wants not to dirty the shirt

When the same person does both actions, Russian often uses:

• чтобы + infinitive

So:

• чтобы не испачкать рубашку = in order not to get the shirt dirty

If the subject were different, Russian would usually use a finite verb instead. For example:

• Я надел фартук, чтобы рубашка не испачкалась
  I put on an apron so that the shirt wouldn’t get dirty

Because испачкать is perfective, and that fits the meaning better here.

The idea is to avoid a result:

• to avoid ending up with a dirty shirt
• not to get the shirt dirty

Perfective verbs often focus on a single completed result, and here the unwanted result is the shirt becoming dirty.

• испачкать = to dirty, to stain, to get something dirty
• пачкать = to dirty, to be dirtying, to dirty repeatedly/in general

So:

• чтобы не испачкать рубашку is the natural choice

Using не пачкать would sound more like in order not to be dirtying the shirt or not to dirty it in general, which is less precise here.

Because it is a feminine noun in the accusative singular.

• nominative: рубашка
• accusative: рубашку

It changes because it is the direct object of испачкать:

• испачкать что? — рубашку

This is a very common pattern for feminine nouns ending in -а:

• книга → книгу
• машина → машину
• рубашка → рубашку

Russian punctuation requires commas here for two reasons:

1. Когда я готовлю is a subordinate clause, so it is separated from the main clause:

   • Когда я готовлю, ...

2. чтобы не испачкать рубашку is also a subordinate clause of purpose:

   • ..., чтобы не испачкать рубашку

So both commas are required:

• Когда я готовлю, я надеваю фартук, чтобы не испачкать рубашку.

Yes, Russian word order is flexible, though some versions sound more neutral than others.

The original:

• Когда я готовлю, я надеваю фартук, чтобы не испачкать рубашку.

is natural and neutral.

You could also say:

• Я надеваю фартук, когда готовлю, чтобы не испачкать рубашку.

That is also understandable and natural.

But word order in Russian often affects focus and style, not just grammar. The original version is a very good standard sentence for learners because it is clear and straightforward.